According to Katrina Berne, Ph.D., "Thyroid hormone controls metabolism by converting calories to energy. Hypothyroidism is common in CFS/FMS patients. Many physicians treat hypothyroidism when symptoms are present and test results are low or low-normal. Hypothyroidism slows the heart rate and is characterized by symptoms of sluggishness, fatigue, modest weight gain, dry skin, hair loss, brittle nails and hair, cold intolerance, irritability, confusion, and depression. Natural replacements supply both thyroxine and triiodothyronine, whereas synthetic thyroids replace thyroxine alone." (Source: Chronic Fatigue Syndrome, Fibromyalgia and Other Invisible Illnesses, by Katrina Berne, Ph.D. Published by Hunter House and available at www.hunterhouse.com or by calling 1-800-266-5592.